The deposition of particles by means of low pressure cold spraying (LPCS) processes, for example on aluminum, steel and other alloy parts, has to date only been possible with the deposition of aluminum, copper and nickel particles having a size range of between about 45-50 μm and providing low adhesion strength and deposition efficiency of about 12-15%. The presence of an oxide layer on the particle surface makes it difficult to form a high adhesion coating of the particles utilizing the LPCS process, while diminishing of the oxide layer thickness allows to deposit coating with higher adhesion and considerably improved coating/substrate interface structure [R. Gr. Maev, V. Leshchynsky, Cold Gas Dynamic Spray, CRC Press, 2016, 340p].
In order to obtain high adhesion coatings with high deposition efficiency by LPCS, the oxide layer has to be maximally altered, diminished or removed from the particle surfaces. There are a few feasible methods for reduction of the oxide film, including a mechanical breakdown, reaction/plasma processing or heat treatment of the particles. For example, for Aluminum powders according to [A. Kimura et al., Reduction mechanism of surface oxide in aluminum alloy powders containing magnesium studied by x-ray photoelectron spectroscopy using synchrotron radiation, Appl. Phys. Lett. 70/26, (1997) 3615-3619], the removal of the oxide layer requires: (a) the presence of a small amount of Mg (over 0.01 mass %) in the reaction area and (b) an activation temperature above 773° K. Taking into account that most of Aluminum alloy powders (for example, the Al 6022 powders) contain 0.45-0.70 mass % of Mg, such powders are suitable for the oxide layer removal by thermal processing. However, a simple removal of the oxide layer from Aluminum-based alloy powders may not be enough, since a natural aluminum oxide coating will be formed again over the metallic Al 6022 powders exposed to the environment. To prevent re-oxidation, direct in situ nitriding of the Al alloy powders can be used to destroy Al oxide film and form a very thin AlN island on the particle surfaces [T. B. Sercombe and G. B. Schaffer, On the role of tin in the nitridation of aluminium powder, Scr. Mater. 55, (2006) 323-328].
Although schematics of hopper-microreactor, which consists of a powder cartridge holder and opening system, reaction vessel with mixing device and powder valve, powder feeder metering disc unit have been disclosed by U.S. Pat. No. 4,808,042, none have proven feasible for adaption to a LPCS application for the purpose of eliminating the oxide film problem. Therefore, it would be desirable to develop a feed assembly capable of solving the problems associated with prior art assemblies while still delivering powder feed in an economical manner.